Typecheck JS files for chrome://help before doing import transition

third_party/closure_compiler/externs/pending_compiler_externs.js

Index: third_party/closure_compiler/externs/pending_compiler_externs.js
diff --git a/third_party/closure_compiler/externs/pending_compiler_externs.js b/third_party/closure_compiler/externs/pending_compiler_externs.js
index 753bebd1425a59a3f156c25ad55334e601f77362..0b3d06e875fad3fd6f3b73a4d1f4148e7b182add 100644
--- a/third_party/closure_compiler/externs/pending_compiler_externs.js
+++ b/third_party/closure_compiler/externs/pending_compiler_externs.js
@@ -9,3 +9,13 @@
  * @param {string=} opt_title The title to give the new HTML document.
  */
 document.implementation.createHTMLDocument = function(opt_title) {};
+
+/**
+ * @see http://www.whatwg.org/specs/web-apps/current-work/#popstateevent
+ * @constructor
+ * @extends {Event}
+ *
+ * @param {string} type
+ * @param {{state: *}=} opt_eventInitDict
+ */
+function PopStateEvent(type, opt_eventInitDict) {}